Reinforced synthetic articles may be formed by resin transfer molding (RTM) techniques or the like by providing a mixture comprised primarily of fluid polyester resin to be transferred into a closed mold containing reinforcement elements where the mixture wets the reinforcement before curing.
Resin transfer molding is an intermediate molding process, i.e. a process between open molding techniques and reaction injection molding (RIM) techniques, which employs low pressure, lower cost equipment with male and female molds to produce high tolerance parts (with two smooth surfaces) at an intermediate rate. Examples of articles suitable for production by RTM processes include: automobile and van body components, bathtub/shower units, fire helmets, utility boats, skateboards, chemical tank covers and business machine enclosures.
Expanded resin transfer molding (ERTM) employs a gas forming reaction within the mold to generate pressure to expand the base resin mass to fill the mold.
In systems using chemical generation of the gas that expands the resin, it is desirable to have a simple, reproducible gas generation reaction which does not require external addition of heat and which does not interfere with the properties of the end product. Ideally, an ERTM process employing a chemical gas generating system would be inexpensive and require inexpensive equipment, would utilize components that are nontoxic, would be capable of achieving a full range of expanded article densities, and would permit a limited period after mixing to allow the fluid resin to come into pressurized wetting contact with the surface of the reinforcement elements before curing in the expanded state thereby wetting the reinforcing elements therein. This "self working" contact between the resin and the reinforcing elements causes the resin to wet the fibres and remove air from the deposited mass.
Finally, it is desirable that such gas-generating components be capable of functioning over a non-critical range of proportions without harming or altering the composition of the product. Unfortunately, to date, for many resin systems, the previously known expansion agents are inadequate in a number of these respects.
For example, in certain known urethane and polyester expansion producing reactions, the ingredients must be combined stoichiometrically within a critical ratio of 1%. This has caused known systems to employ accurate and expensive pumps, e.g. gear pumps, to deliver the ingredients at a smooth, precise and constant flow rate.
Furthermore, these known foaming reactions occur so quickly that it is not possible to wet the reinforcing fibres within the mold or to force out the air or produce a shrink resistant product that is cured in the expanded state. These systems are therefore not generally useful in producting expanded articles by RTM processes.